Hetero junction bi-polar transistors (HBTs) are bi-polar transistors in which an emitter-base junction is a hetero junction using a material greater in band gap than a base layer for an emitter layer in order to enhance the emitter injection efficiency, and since HTB is suitable as semiconductor devices used in the frequency area higher than microwave band, they are expected to be used as semiconductor devices for portable telephones of the next generation.
The structure of HBT is as follows. In the case of, for example, a GaAs-based HBT, generally an n+-GaAs layer (sub-collector layer), an n-GaAs layer (collector layer), a p-GaAs layer (base layer), an n-InGaP layer (emitter layer) and an N-GaAs layer (sub-emitter layer) are in turn grown as crystal on a semi-insulated GaAs substrate by a Metal Organic Chemical Vapor Deposition (MOCVD), thereby forming a thin film crystal wafer having the above-mentioned layer structure where a pn junction which is an emitter-base junction is of a structure of a hetero junction, and an HBT is produced using the resulting wafer.
FIG. 7 schematically shows a structure of the conventional general GaAs-based HBT. In the HBT 100 of FIG. 7, a sub-collector layer 102 comprising an n+-GaAs layer, a collector layer 103 comprising a n-GaAs layer, a base layer 104 comprising a p-GaAs layer, an emitter layer 105 comprising an n-InGaP layer, a sub-emitter layer 106 comprising an n+-GaAs layer and an emitter contact layer 107 comprising an n+-InGaAs layer are in this turn formed as semiconductor thin film crystal layers on a semi-insulated GaAs substrate 101 by a suitable vapor growth method such as MOCVD method, and a collector electrode 108, a base electrode 109 and an emitter electrode 110 are formed on the sub-collector layer 102, the base layer 104 and the emitter contact layer 107, respectively.
In an HBT constructed as above, the current gain β is shown by β=Ic/Ib=(In−Ir)/(Ip+Is+Ir), wherein In denotes an electron injection current from emitter to base, Ip denotes a hole injection current from base to emitter, Is denotes an emitter/base interface recombination current, and Ir denotes a recombination current in the base.
Therefore, it can be seen from the above formula that in order to increase the current gain β, it is necessary to decrease Ir which is a recombination current in the base. This recombination current in the base is sensitive to the crystallinity of the base layer, and when there are many crystal defects in the base layer, the recombination current in the base increases, resulting in decrease of the current gain β. Thus, in order to improve characteristics of current gain of HBT, it is necessary to make the base layer have a good crystallinity.
As one of the conventional technologies to attain the above object, JP-A-3-110829 proposes a method for producing a compound semiconductor thin film in which the substrate temperature during the growth of the compound semiconductor thin film is set in the range of 450-650° C. and the feeding molar ratio of the raw material of Group V and that of Group III is set in the range of 0.3-2.5.
According to the above proposed conventional method, it is disclosed that the carrier concentration can be controlled to 1×1018 cm−3−1×1020 cm−3, but the method has a problem that when the feeding molar ratio of the raw material of Group V and that of Group III and the growth temperature are determined, the carrier concentration is determined thereby and thus it is difficult to control the carrier concentration to a desired value.